Many manufacturing processes require the automated loading and unloading of workpieces into and out of a processing apparatus. In the interest of reducing cost and increasing productivity, such movement of workpieces is often accomplished with the aid of a workpiece handling end-effector (sometimes referred to as a “wand”) on the end of a robotic arm.
One example of such a manufacturing process is the planarization of a surface of a workpiece, a process that finds application in the manufacture of many types of products such as semiconductor wafers, optical blanks, memory disks, and the like. Chemical mechanical planarization (CMP) is one accepted method for achieving a planar surface on such workpieces. The CMP method typically requires the workpiece to be loaded into and mounted precisely on a carrier head in a manner such that the surface to be planarized is exposed. The exposed side of the workpiece is then held against a polishing pad and a relative motion is initiated between the workpiece surface and the polishing pad in the presence of a polishing slurry. Typically the workpieces are processed in batches or lots that include a plurality of workpieces. For example, with the CMP processing of semiconductor wafers, each of the wafers in a lot must be sequentially loaded from a wafer cache onto the carrier head for planarization. Following the planarization, each wafer is unloaded from the carrier head and again placed in a wafer cache or is directly transferred to a subsequent processing apparatus such as a cleaning station. The loading and unloading operations are accomplished using a wafer handling end-effector.
With many workpieces, and certainly with semiconductor device wafers, the surfaces of the workpieces can be easily damaged if the handling end-effector contacts the surface. Because of this, the end-effector should preferably contact only the edge of the workpiece or, or at most, the surface within a narrow distance from the edge. This requirement is made even more significant by the current migration of the semiconductor industry from 200 mm (8 inch) to 300 mm (12 inch) wafers. As part of this change, the semiconductor industry has adopted new wafer-handling standards for 300 mm wafers that preclude all contact with the major surfaces of a wafer, and tighten limitations as to the extent of the wafer that may be contacted at the wafer edge. Thus, known vacuum type end effectors, or other end effectors that grip or touch the back surface of the wafer are not allowed. Also, the available surface area for fabrication of the electronic devices on a 300 mm wafers is more than double that of 200 mm wafers. The cost of 300 mm wafers increases proportionately to the increase in wafer area, placing a premium on the reliability and safety of wafer handling systems and end-effectors. In addition, 300 mm wafers are proportionally heavier than 200 mm wafers, adding still more demands on the mechanical integrity, precision, and reliability of the end-effector. Existing end-effector designs do not satisfactorily meet these new design and reliability requirements for 300 mm wafers.
Loading a workpiece into a chemical mechanical planarization apparatus presents problems for conventional workpiece handling end-effectors because of the nature of the CMP carrier head. The conventional CMP carrier head includes a flexible diaphragm against which the back surface (the surface that is not to be polished) is pressed. The flexible diaphragm is surrounded by an annular wear ring or retaining ring having an inner diameter only slightly greater than the diameter of the workpiece to be polished. The diaphragm and the wear ring form a cavity into which the workpiece must be loaded. To carry out the planarization operation, the workpiece must be mounted against the diaphragm within the confines of the wear ring while contacting only the edges or near edge surfaces of the workpiece. In the CMP processing of a 200 mm (8 inch) or 300 mm (12 inch) semiconductor wafer the clearance between the inner diameter of the wear ring and the outer diameter of the semiconductor wafer is typically less than 1 mm. The recess into which the semiconductor wafer must be loaded has a depth on the order of the thickness of the wafer itself, or about 0.75 mm.
Many other types of processing apparatus also require a workpiece to be loaded into a recessed space with a high degree of positional accuracy and without adversely contacting the surfaces of the workpiece. Accordingly, there exists a need for a workpiece handling end-effector that can load workpieces into and unload workpieces from a workpiece processing apparatus with a high degree of precision and without adversely contacting the critical surfaces of the workpiece. There also exists a need for a method for processing workpieces in an efficient manner using such a workpiece handling end-effector.